DESCRIPTION (Applicant's Abstract): The introduction of protease inhibitors has led to considerable optimism among patients and providers alike, but it has been shown that unless patients strictly adhere to their medication regimen, viral replication will increase and protease inhibitor--resistant viral mutations will arise. It is therefore of the utmost importance that the causes of non-adherence be identified and eradicated. One potential cause of non-adherence, particularly in older HIV+ patient, which has yet to be adequately addressed, is neuropsychological dysfunction. The primary aim of this study is to determine the effects of neuropsychological impairment, particularly memory deficits (prospective, retrospective, and working), dual task/executive dysfunction, and depression/apathy on HIV+ patients; adherence with their protease inhibitor regimen. A corollary aim is to ascertain how older age potentiates the deleterious effects of HIV- associated cognitive decline on adherence. The secondary aim is to determine the independent and interactive effects of advancing age and HIV infection on neurocognitive and neuropsychiatric function. The central hypothesis of the study is that HIV-associated cognitive impairment will lead to poor treatment adherence and that relationship will be magnified in older patient. Methods: 200 HIV seropositive participants (1/2 over age 50 years) will be administrated: (1) a battery of neuropsychological tests (both computerized measures of formation processing and clinical measures of memory and executive function) that have been chosen because of their hypothesized sensitivity to the effects of HIV infection and the normal aging process; (2) Measures of psychological function; and (3) Measures of health beliefs, intentions and related constructs which have been shown to predict adherence. The HIV+ participants; adherence to protease inhibitors will then be monitored for 3 months. Medication Event Monitoring System (MEMS) caps, which employ a microprocessor to automatically record the date and time each dose is taken will be the "gold standard" for indexing adherence. Given the multi-determined nature of adherence as well as the complex data set this study will yield structural equation modeling using EQS will be the primary analytic strategy employed. Increased understanding as to how neuropsychological dysfunction impacts on treatment adherence, particularly among older HIV+ patients, is of critical importance given the personal and public health consequences of non-adherence.